JP-2004-301165A shows a variable valve timing controller (VVT) which varies a rotational phase of a camshaft relative to a crankshaft of an internal combustion engine. The VVT is provided with a variable-camshaft-timing (VCT) mechanism which adjusts a rotational phase of an intake camshaft by using of a differential hydraulic pressure between a pressure in an advance chamber and a pressure in a retard chamber; and an oil flow control valve (OCV) which controls the differential hydraulic pressure. The OCV is one example of the electromagnetic spool valve.
The OCV includes a spool valve having four-way valve structure, and an electromagnetic actuator (linear solenoid) which drives the spool. Referring to FIG. 8, the conventional electromagnetic actuator will be described.
The electromagnetism actuator 1 has a plunger 3 and a stator 4 between which a magnetic attracting force is generated at multiple portions thereof. The plunger 3 can axially slide relative to the stator 4 from a position where the plunger 3 does not overlap with the stator 4 to another position where the plunger 3 overlaps with the stator 4.
In order to generate the magnetic attracting force at multiple portions, the plunger 3 has a ring-shaped outer plunger-projection 53 and a ring-shaped inner plunger-projection 54, and the stator 4 has a ring-shaped stator-projection 52d which is able to slide in between the outer and inner plunger-projections 53 and 54. Thereby, the magnetic attracting force is generated at a portion “X” between the outer plunger-projection 53 and the stator-projection 52d. Further, the magnetic attracting force is generated at a portion “Y” between the inner plunger-projection 54 and the stator-projection 52d. 
As above, since the magnetic attracting force is generated at two portions “X” and “Y”, a magnetic efficiency can be enhanced when starting to drive the plunger 3.
However, in the above configuration, the portion “X” is established when an inner circumferential wall of the outer plunger-projection 53 overlaps with an outer circumferential wall of the stator-projection 52d. Thus, as shown by a long dashed short dashed line “X” in FIG. 8B, as the plunger 3 comes closer to the stator 4, the magnetic attracting force is decreased at the portion “X”.
Similarly, the portion “Y” is established when an outer circumferential wall of the inner plunger-projection 54 overlaps with an inner circumferential wall of the stator-projection 52d. Thus, as shown by a dashed line “Y” in FIG. 8B, as the plunger 3 comes closer to the stator 4, the magnetic attracting force is decreased at the portion “Y”.
In the following description, the characteristic in which the magnetic attracting force is decreased as the plunger 3 comes closer to the stator 4 is referred to as a parabolic characteristic.
The plunger 3 receives a resultant force of the magnetic attracting forces generated at the portions “X” and “Y”. After the plunger axially overlaps with the stator 4, the magnetic attracting force applied to the plunger 3 is decreased as the plunger comes closer to the stator, as shown by a solid line “G” in FIG. 8B. That is, although the magnetic efficiency can be enhanced at starting of driving the plunger 3, the magnetic attracting force is significantly decreased after the plunger 3 and the stator 4 overlap with each other.